Inelastic neutron scattering and raman light scattering from hydrogen-filled clathrates hydrates

Several samples of ternary tetrahydrofuran-H2O-H2 and binary H2O-H2 clathrate hydrates have been analysed by high-resolution inelastic neutron scattering and Raman light scattering. The neutron spectrum presents several intense bands due to H2 molecule excitations and in particular to rotational transitions, centre-of-mass translational transitions of either para-or ortho-H2, and to combinations of these. The H2 molecule behaves in the clathrate cage as an almost free rotor, and performs a translational motion (rattling), that is a paradigmatic example of the motion of a quantum particle in a non-harmonic three-dimensional potential well. Both the H2 rotational transition and the fundamental of the rattling transition split into triplets. Raman spectra show a similar splitting of the S0(0) rotational transition, due to a significant anisotropy of the potential with respect to the orientation of the molecule in the cage. The comparison of our experimental values for the transition frequencies to a recent quantum mechanical calculation is discussed.